Torque meters are torque-measuring devices that exist in multiple formats depending on the field of application, the equipment for which the torque must be measured and the degree of accuracy required. In the aeronautical field, hydraulic torque meters are commonly used. These torque meters operate, for example, by means of an intermediate helical-toothed gear, for which the torque applied results in an axial thrust proportionate to the torque to be measured. This axial thrust is applied to a piston connected to the intermediate gear. Another embodiment commonly encountered involves measuring the axial thrust of a ring gear of an epicyclic gear train, which is also proportionate to the torque to be measured. In more general terms, the measurement can be taken on any helical gear.
The movement of the piston according to the torque measured causes a pressure to be exerted on the oil in a measuring chamber supplied by a hydraulic circuit until a balance is reached between the pressure exerted by the piston on the oil and that exerted by the oil on the piston. The measurement of the pressure at this balance by a pressure sensor allows the measured torque to be deduced.
These hydraulic torque meters have the drawbacks of being sensitive to friction due to the use of sealing and bearing elements, to leaks caused by the ageing of the sealing elements, to the variation in oil viscosity, and to the temperature, etc. Therefore, the accuracy of hydraulic torque meters is not guaranteed and regular maintenance thereof is required. Moreover, a negative torque cannot be measured using such torque meters.
Other torque meters used include rotary torque meters comparing the phase angle of the shaft subject to the torque with a rotating shaft not subject to the torque. However, the accuracy of the torque measurement taken by rotary torque meters depends on the length of the shaft, which thus requires rotary torque meters to be particularly bulky in order to produce an accurate measurement. Moreover, the rotating shaft not subject to the torque, and used as a reference, represents an additional cost and weight for the system.